In the development of various types of implantable biomedical apparatus, such as cardiac pacemakers and other implantable body tissue stimulators, it has previously been proposed that a rechargeable battery or cell be utilized to supply operating energy to the remaining circuitry of the implanted unit. By way of examples, in my earlier issued U.S. Pat. Nos. 3,867,950; 3,888,260; and, 4,096,866, I disclosed the use of a rechargeable nickel-cadmium cell for supplying operating energy to implantable cardiac pacemaker and tissue stimulator circuitry. As disclosed in these prior patents, the use of such a rechargeable battery is intended to obviate the necessity of having to replace depleted batteries, as often occurs when non-rechargeable type batteries are employed for such implantable application.
The need remains, however, to improve the control of the recharging operation for such an implanted rechargeable cell, in order to assure that the recharging operation is performed in an efficient, yet safe manner. If has previously been proposed, for example, to provide a means for sensing when a rechargeable battery, such as the nickel-cadmium cell, has been returned to its fully charged condition, during a recharging operation, for controlling the recharger in such a manner as to discontinue charging when full-charge state is achieved. As pointed out by Frezzolini et al in their U.S. Pat. No. 3,775,661, nickel-cadmium cells produce no discernible repetitive voltage characteristics, thus making battery potential a relatively ineffective state-of-charge indicator for use in controlling the re-charging operation. Instead, this prior art suggests that a better indication of the state-of-charge of a nickel-cadmium cell is the increase in gaseous pressure within the cell casing which enclose the chemical elements of the battery, produced by the buildup of oxygen therein when the cell is returned to its fully charged state. Frezzolini et al, however, require the provision of a vent or aperture through the cell casing which communicates internal cell pressure build-up with a resilient diaphragm disposed in a control unit mounted external to the battery. As the pressure within the cell casing increases, the diaphragm is flexed to actuate an associated switch connected in the battery charging circuitry, so as to de-energize the charger when the battery internal pressure indicates a return to the fully charged state.